Organ motion and setup error are major causes of target position uncertainty in external radiotherapy for extracranial targets. Uncertainty of target position and shape can result in decreased radiation dose to the target and an increased dose to the surrounding normal tissues. To compensate for the uncertainty of the target position and shape in irradiation process the planning target volume (PTV) must have a larger margin compared to static targets with the same clinical target volume (CTV). This approach increases the probability that the target will receive a lethal dose of radiation. Unfortunately, it also increases collateral damage to surrounding healthy tissues. Some approaches resort to using a learned target motion trajectory model from a 4D CT in the planning phase to guide the radiation beam in the treatment phase, which obviously has the drawback of mismatch between the model trajectory and the actual target motion trajectory.
U.S. Pat. No. 6,804,548 (Takahashi et al.) is directed at a system and method for monitoring irradiation target movement and recognizing irradiation target position in an irradiation process without implanted fiducial markers. The method disclosed in '548 employs a high resolution 3D imaging device such as CT or MRI for taking two sets of 3D images of the irradiation target region (one set, denoted by H0, in radiation planning phase, and another set, denoted by H1, in the treatment phase immediately before the start of irradiation). The method disclosed in '548 uses mutual information measure to compare the irradiation target region extracted from H0 and irradiation target region extracted from H1. As a result of the comparison, the method computes the matching between H0 and H1, and obtains a transformation function incorporating the changes in the irradiation conditions from the irradiation plan. The method disclosed in '548 also employs a real-time imaging device such as an echograph for taking images of the irradiation target region immediately before the start of irradiation and also during the irradiation procedure. Note that for small targets echograph may not be an ideal imaging modality. The real-time image taken immediately before the start of irradiation is denoted by R0. The real-time images taken during the irradiation procedure are denoted by Rn. The high resolution image set H1 and real-time image R0 are taken at nearly the same time but with different modalities. The high resolution image set H1 is then superimposed with the real-time image R0 so that the new irradiation plan is reflected in the real-time image R0. During the irradiation treatment, the method '548 compares the real-time images Rn and R0. According to the result obtained by the comparison, the method identifies the portion in the real-time image Rn, which corresponds to the irradiation target in the real-time image R0, extracts the target region in the real-time image Rn and computes the position and direction in the 3D coordinate system in the work space for measurement. The method makes a decision as to whether the target tumor is present or not in the region expected in the irradiation plan. To compute the exact position of the target, the method disclosed in '548 installs 3D position and direction sensors such as infrared rays, ultrasound, or magnet on the treatment table, real-time imaging device and high resolution imaging device to find relative position and direction between them.
Marker-less target position monitoring during radiation treatment is a needed feature for radiotherapy to increase accuracy and mitigate damages to normal tissues. However, it is known that extracranial targets may change their shape due to tissue deformation during a bodily process, e.g. a respiration cycle. Furthermore, targets may shrink after a number of fractions of treatment. Therefore, shape tracking is also very much desirable especially for conformal radiation by using automatic beam shaping device such as a multi-leaf collimator (MLC).
Methods of using position and direction sensors to find relative position and orientation of imaging devices to a reference 3D system only solves the problem of extrinsic parameters (e.g. position and orientation of the device with respect to a 3D radiotherapy system) estimation for these devices. To compute 3D position for the target region, the intrinsic parameters (e.g. distance from an X-ray source to a detector and pixel pitch of the detector) of an imaging device must be provided. Intrinsic parameters of an imaging device may be obtained from the device specification sheets and on-site installation specs. However, intrinsic parameters are largely device dependent. For instance, the distance from an X-ray source to X-ray detector can change from device to device within a certain statistical range for the type of devices.
U.S. Patent Application Publication No. 2005/0180544 A1 (Sauer et al.) discloses a system and method for patient positioning for radiotherapy in the presence of respiratory motion. The method disclosed in '544 teaches using one or two X-ray imagers to acquire two sequences of the region that contains the target with an invasively implanted marker. If one X-ray imager is used, images are taken alternatively at two different angles (0° and 90°). The frequency of the image acquisition within a sequence is fixed and trigged by a respiratory monitoring device (noted that the fixed frequency mode may not be ideal because the organ motion caused by respiration is non-linear in nature in terms of 3D positions). After obtaining two sequences of X-ray images of the target region, method '544 teaches using the triangulation scheme to compute 3D coordinates of the target to form a 3D target position trajectory. The 3D target position trajectory enables radiation treatment with beam tracking or gating, thus allowing for motion compensation for all fields in which radiation doses are being delivered. In method '544, the 3D target position is also compared with a 4D target position data in a sequence of CT images to see if they match. If there is a significant change in the planned treatment, the treatment is stopped.
People skilled in the art understand that to effectively use the triangulation scheme, the intrinsic parameters of the X-ray imagers must be given, which is the same drawback that method '548 has. Noted also that method of '544 teaches manually or semi-automatically identifying targets in the captured images, which is not desirable in real-time irradiation adaptation.
The present invention is designed to overcome the problems set forth above.